Title: Periodic Mesoporous Hexagonal Boron Nitride at High Pressure: A Route to Cubic Boron Nitride Nanocrystals and Mesoporous Cubic Boron Nitride

Periodic mesoporous hexagonal boron nitride was investigated as a precursor for nanoscale forms of cubic boron nitride. Cubic boron nitride (c-BN) nanocrystals were obtained at a pressure of 14 GPa and a temperature of 1300 degrees C. The synthesized nanocrystals have diameters of similar to 50 nm and are solutionprocessible. Mesoporous c-BN formed at a pressure of 10 GPa and 1000 degrees C. The mesoporous c-BN has a pore size distribution centered around 3 nm and a surface area of 122 m(2)/g as determined by electron tomography. At pressures of 8 and 6 GPa near monodisperse h-BN nanodiscs with diameters of 0.5-1 mm and 0.2-0.5 mm were formed, respectively. The discs formed colloidal solutions in acetone. The hydrodynamic radius of the discs matched the radii determined by dynamic light scattering indicating absence of nanodisc aggregation in solution.